The present invention relates to the use of electromagnetic radiation to move droplets of fluid on a fluid-transporting surface. In particular, the invention relates to microfluidic devices and methods in which radiation is directed through a material that is substantially transparent to that radiation in order to optically move fluid droplets on a fluid-transporting surface. Typically, optical movement of fluid droplets is achieved while evaporative loss from the droplets is reduced.
A number of factors have contributed to the recent advances in the fields of biological sciences, biochemical assays, clinical diagnostics, and synthetic and analytical chemistry. These factors, for example, include the growing significance of genomics, the emergence of proteomics, and developments in combinatorial chemistry. In addition, the rise of drug-resistant forms of infectious diseases, increases in the incidence of food-chain contamination by pathogenic bacteria, the threat of biological warfare, and the continued prevalence of infectious diseases in underdeveloped countries also highlight the need for improved techniques for drug screening, drug target validation, toxicology studies, and combinatorial chemistry.
In these fields, progress has often been limited by the inability to process large numbers of samples at high speed. Extended time frames are necessitated by tedious sample preparation techniques and slow detection methods. These constraints in turn make automation of diagnostic assays difficult and create barriers to driving assay costs down. Thus, there is a current need in the art for rapid and inexpensive techniques to carry out diagnostics, parallel syntheses, and high throughput screening.
Expensive or rare fluids are employed in many emerging scientific applications, such as proteomics and genomics. Thus, considerable interest has been focused on microfluidic techniques, which typically involve small sample volumes and low reagent consumption. In addition, microfluidic techniques may be used to carry out numerous parallel processes, can be used across a range of fluid properties, and are compatible with movement of biological moieties that may vary by orders of magnitude in size and physical characteristics (e.g., from peptide hormones to intact cells). Processes in a microfluidic format are, therefore, particularly amenable to automation, enabling routine screening and surveillance programs to be established. In addition, new process paradigms, such as flow-through processing of biological samples, become feasible only in a microfluidic format.
A variety of microfluidic devices have been developed for chemical and bioanalytical applications. Typically, microfluidic devices involve the miniaturization and automation of a number of laboratory processes, which are then integrated on a chip. Thus, microfluidic technology may be employed to carry out a series of chemical or biochemical processes in a single device, including sample purification, separation, and detection of specific analytes. Applications include medical diagnostics, genetic analysis, or environmental sampling. See, e.g., Ramsey et al. (1995) xe2x80x9cMicrofabricated chemical measurement systems,xe2x80x9d Nat. Med. 1:1093-1096.
Microfluidic devices may be constructed using simple manufacturing techniques and are generally inexpensive to produce. For example, the microfabrication methods used to make microchips in the computer industry may also be used to create microfluidic devices, enabling the creation of intricate, minute patterns of interconnected channels. Once a pattern is created, microchip manufacturing methods are employed to recreate the channel design in a substrate. In some instances, chemical etching or stamping techniques are employed. As a result, highly precise channels with dimensions that can be varied in their width and depth may be produced on a substrate. Once the pattern is produced in the substrate, a cover plate is affixed over the substrate so as to form conduits in combination with the channels.
Typically, the substrates and/or cover plates are comprised of a rigid material such as glass (see, e.g., Woolley et al. (1994), xe2x80x9cUltra-high-speed DNA fragment separations using microfabricated capillary array electrophoresis chips,xe2x80x9d Proc. Natl. Acad. Sci. USA 91:11348-11352), plastic (see, e.g., McCormick et al. (1997), xe2x80x9cMicrochannel electrophoretic separations of DNA in injection-molded plastic substrates,xe2x80x9d Anal. Chem. 69:2626-2630), silicon, or quartz. Alternatively, microfabricated elastomeric valve and pump systems have been proposed in International Patent Publication No. WO01/01025. Similar valves and pumps are also described in Unger et al. (2000) xe2x80x9cMonolithic microfabricated valves and pumps by multilayer soft lithography,xe2x80x9d Science 288:113-116. These publications describe soft lithography as an alternative to silicon-based micromachining as a means by which to form microfluidic devices. Through soft lithography, microfluidic structures created entirely from an elastomer may be constructed containing on/off valves, switching valves, and pumps.
The above-described microfluidic devices, however, pose certain technical challenges that must be overcome. For example, fluid flow characteristics within the small flow channels of a microfluidic device may differ from the flow characteristics of fluids in larger devices, since surface effects tend to predominate, and regions of bulk flow become proportionately smaller. Consequently, several techniques have been developed in order to achieve fluid flow control in microfluidic devices. One commonly used technique involves the generation of electric fields to manipulate buffered, conductive fluids around networks of channels through electrophoretic or electroosmotic forces. See, e.g., Culbertson et al. (2000), xe2x80x9cElectroosmotically induced hydraulic pumping on microchips: differential ion transport,xe2x80x9d Anal. Chem. 72:2285-2291. Another technique, as described in Anderson et al. (2000), xe2x80x9cA miniature integrated device for automated multistep genetic assays,xe2x80x9d Nucleic Acids Res. 28:E60, involves achieving fluidic control by coupling the device to an external system of solenoid valves and pressure sources.
The use of three-dimensional channels to define fluidic pathways, however, gives rise to several limitations. For example, leakage of fluids or analytes into undesired channels through diffusion or the influence of local electric field gradients requires the precise control of bias voltages along each channel in the microfluidic network. This need for precise control increases the design constraints for complex systems and requires that a high degree of engineering sophistication be incorporated into the microfabricated device. In particular, electric field gradients and path length differences around bends or corners in fluidic channels will also distort the distribution of analytes within the sample stream. This distortion can degrade the performance of an electrophoretic separation. In addition, the infrastructure and circuitry required to establish the electric fields are associated with certain spatial requirements that limit the complexity of the microfluidic devices. Furthermore, microvalves and other fluid control mechanisms greatly increase the complexity, cost, and manufacturability of such highly integrated designs.
In some instances, electric fields may be applied to move fluids without the use of three-dimensional channels. For example, U.S. Pat. No. 6,294,063 to Becker et al. describes microfluidic devices that programmably manipulate packets of fluids through the application of electric fields via electrodes located on the devices. A fluid is introduced onto a reaction surface and compartmentalized to form a packet. An adjustable programmable manipulation force is applied to the packet according to the position of the packet. As a result, the packet is programmably moved according to the manipulation force. In some cases, electromagnetic radiation may be used to maintain photochemical reaction or for sensing processes.
An alternative technique uses electric fields to effect fluid movement without channels and relies on the ability of electric fields to change the contact angle of a fluid on a surface (see, e.g., Lee et al. (2002), xe2x80x9cElectrowetting and electrowetting-on-dielectric for microscale liquid handling,xe2x80x9d Sensors and Actuators A 95:259-268). Upon application of an electric field gradient to a droplet on a fluid-transporting surface, different contact angles are formed between leading and receding surfaces of the droplet with respect to the fluid-transporting surface. This imbalance in surface tension forces will produce a net force, and move the droplet. Other techniques have been explored for manipulating droplets of liquid on a substrate surface. For instance, Ichimura et al. (2000), xe2x80x9cLight-driven motion of liquids on a photoresponsive surface,xe2x80x9d Science 288:1624-1626, describes placing a macroscopic droplet upon a particular surface polymer layer on a substrate, and using ultraviolet or blue light to change the isomeric form of the polymer layer. As a result, the contact angle between the droplet and the surface is changed, thereby moving the droplet. This approach is currently incapable of moving fluids at speeds exceeding about 35 xcexcm/s, however, and it does not work with polar fluids.
Another technique involves the use of thermal gradients to move droplets of liquid. See, e.g., Kataoka et al. (1999), xe2x80x9cPatterning liquid flow on the microscopic scale,xe2x80x9d Nature 402:794-797. The application of thermal gradients generates a thermocapillary shear stress at the air-liquid interface that is capable of driving a droplet across a surface. By the patterning of channels onto a surface, fluid movement can be precisely controlled. Another thermal gradient-driven approach, as described in Daniel et al. (2001), xe2x80x9cFast droplet movements resulting from the phase change on a gradient surface,xe2x80x9d Science 291:633-636, uses droplet coalescence to obtain droplet speeds of over 1 m/s. In either of these cases, however, the application of heat may not be suitable for some aspects of biological assays, and may exacerbate evaporation problems.
Optical trapping is a widely exploited phenomenon for manipulating items such as atoms, molecules, and small particles. The fundamental principle behind optical trapping is that light carries momentum, which can then be expressed as radiation pressure. When light is absorbed, reflected, or refracted by a material, momentum is transferred to the material. Optical tweezers have been developed that controllably deliver radiation pressure to manipulate small particles. Exemplary applications of optical tweezer technology include the manipulation of biological particles, such as cells, bacteria, and viruses (see, e.g., U.S. Pat. No. 4,893,886 to Ashkin et al. and U.S. Pat. No. 6,067,859 to Kas et al.), and the immobilization of biomolecules such as DNA, RNA, proteins, lipids, carbohydrates, or hormones (see U.S. Pat. No. 6,139,831 to Shivashankar et al.). While U.S. Pat. No. 6,294,063 to Becker et al. alludes to the use of optical tweezer technology for microfluidic manipulation, the microfluidic devices described require electrodes in order to apply electric fields that move the droplets and/or sense the position of the droplets. Since electrodes are generally opaque and do not allow the transmission of light therethrough, the microfluidic devices described in Becker et al. are generally incompatible with optical tweezer technology. As a result, optical trapping has not been successfully implemented in the field of microfluidics.
Thus, there exist opportunities in the field of microfluidics to employ optical techniques in order to controllably move fluids on a substrate surface.
In a first embodiment, an optical microfluidic device is provided, comprising a solid substrate, an electromagnetic radiation source, and a means for directing electromagnetic radiation. The solid substrate has a fluid-transporting surface and an opposing electromagnetic radiation-receiving surface, and is comprised of a material that is substantially transparent to electromagnetic radiation of a particular wavelength. The electromagnetic radiation source serves to generate electromagnetic radiation having the particular wavelength. The means for directing electromagnetic radiation is adapted to direct radiation generated by the source toward the radiation-receiving surface of the substrate. As a result, the directed electromagnetic radiation is transmitted through the substrate to the fluid-transporting surface in a manner effective to optically move one or more droplets of a selected fluid on the fluid-transporting surface from a first site to a second site.
In another embodiment, the invention provides an optical microfluidic device as above, except that the solid substrate is not necessarily comprised of a substantially transparent material. In such a case, an optically homogeneous cover plate is provided, comprising a material that is substantially transparent to electromagnetic radiation of the particular wavelength. In addition, the cover plate has an electromagnetic radiation-receiving surface and an opposing surface that faces the fluid-transporting surface. Accordingly, the means for directing electromagnetic radiation is adapted to direct radiation generated by the source toward the radiation-receiving surface of the cover plate. The directed electromagnetic radiation is transmitted through the cover plate to the fluid-transporting surface of the substrate in a manner effective to optically move one or more droplets of a selected fluid on the fluid-transporting surface from a first site to a second site.
In another embodiment, the invention provides an optical microfluidic device as generally described above except that neither a substantially transparent substrate or cover plate is required. Instead, the inventive device is comprised of a substrate (transparent or opaque) having a fluid-transporting surface, an electromagnetic radiation source, and a means for reducing evaporative loss of any droplet or droplets on the fluid-transporting surface.
In yet another embodiment, a method is provided for controllably moving a droplet of a selected fluid across a surface of a solid substrate. A droplet of a selected fluid is deposited on a first site of a fluid-transporting surface of a solid substrate. The substrate is comprised of a material that is substantially transparent to electromagnetic radiation of a particular wavelength. Then, electromagnetic radiation having the particular wavelength is directed toward a radiation-receiving surface that opposes the fluid-transporting surface of the substrate. As a result, the directed radiation is transmitted through the substrate to the fluid-transporting surface in a manner effective to optically move the droplet to a second site on the fluid-transporting surface.
In a further embodiment, a method is provided for controllably moving a droplet of a selected fluid across a surface of a solid substrate as described above, except that the substrate may not be substantially transparent. Instead, an optically homogeneous cover plate is provided comprising a substantially transparent material and having an electromagnetic radiation-receiving surface and an opposing surface. The cover plate is placed over the fluid-transporting surface, such that the opposing surface faces the fluid-transporting surface. Then, electromagnetic radiation is directed toward the radiation-receiving surface and through the cover plate to the fluid-transporting surface in a manner effective to optically move the droplet to a second site on the fluid-transporting surface.
In a still further embodiment, the invention provides methods for controllably moving a droplet of a selected fluid across a surface of a solid substrate as described above, except that the method additionally involves reducing evaporative loss from the droplet while the droplet is on the fluid-transporting surface. In this embodiment, the substrate may or may not be substantially transparent to light of the particular wavelength.
In yet another embodiment, the invention provides a method for controllably moving a fluid droplet across an interior surface of an enclosure, wherein the enclosure is formed at least in part by an optically homogeneous solid wall comprised of a material that is substantially transparent to electromagnetic radiation of a particular wavelength. A droplet of a selected fluid is deposited on the interior surface of the enclosure at a first site. Then, electromagnetic radiation of the particular wavelength is directed through the solid wall in a manner effective to optically move the droplet across the interior surface to the second site.